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Useful Notes on Sex-Linked Inheritance | Biology (853 Words)

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Useful notes on Sex-Linked Inheritance!

Genes other than sex determiners are also located on the ‘X’ chromosomes. Many mutant genes in human have been shown to be located on X chromosome. The inheritance of such genes are called sex-linked inheritance.

The first evidence for sex linkage in a particular species was given by T.H. Morgan in 1910 in white-eyed mutant in Drosophila. When white-eyed male was mated with a red-eyed female the F1 flies were all red-eyed. F2 generation of it included 3: 1 ratio of red and white-eyed flies. But all white eyed flies of F2 generation were males only. When normal female of F1 is crossed with normal male 50% of males were white-eyed and 50% were red-eyed (Fig. 46.17). It shows that the recessive allele is expressed in male only.

Morgan explained it as the association of the gene on ‘X’ chromosome. Because male has only one X chromosome all characters present on it will be expressed. The mutant allele (white eye) of the male is transferred to the female (heterozygous). The hemizygous males obtain their X chromosome from their heterozygous mother only then they will be 50% red-eyed (w+) and 50% white eyed (w).

The female can be white-eyed (ww) only when both ‘X’ carry gene for it. When heterozygous female (w+) is crossed with white-eyed male (w) then white-eyed females can be obtained (Fig. 46.18).

‘Y’ linked inheritance:

Most sex-linked genes in male (heterogametic) are on ‘X’ chromosome. But a few genes are present on ‘Y’ chromosome also. These are called ‘Y’ linked or holandric genes. They are transmitted from father to son directly.

‘Y’ chromosome in Drosophila is mostly heterochromatic. The X chromosomes with many genes and ‘Y’ with virtually none establish the characteristic pattern of inheritance for sex-linked traits. The mother with two ‘X’ chromosomes transmit one X to each gamete. Zygotes that receive a Y chromosome from father, develops into male.

The hemizygous male progeny expresses the sex- linked genes received from the mother. The expression of sex-linked traits in females follow the same pattern as autosomal traits with the recessive phenotype appearing only in homozygous condition.

‘X’ linked traits in human:

The inheritance pattern associated with sex linkage was recorded by Greek philosopher for the first time. An inherited character was observed in a father but not in any of his children (either male or female) and then it reappears in the males of next generation. It is the criss cross pattern, from father to daughter to grandson. It is the ‘X’ linked inheritance. (Fig. 46.19).

(b) Traits are transmitted from an affected man through his daughter to half of his grandsons.

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(c) An X-linked allele is never transmitted directly from father to son.

(d) All affected females have an affected father and a carrier or affected mother.

Dominant X-linked traits:

Males expressing the trait would be expected to transmit it to all of their daughters but none of their sons. Heterozygous females would transmit the trait to half of their children of either sex. If a female expressing the trait is homozygous, all her children will inherit the trait. Sex linked dominant inheritance cannot be distinguished from autosomal inheritance in the progeny of females expressing the trait, but only in the progeny of affected males.

Sex-linked disease hemophilia and pedigree analysis in European royalty which generally showed transmission through females who were unaffected to some of their male offspring who were hemophilic. The disease starts with Queen Victoria of England. Who passed the hemophilic gene to 3 of 9 children half of her son were hemophilic.

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Queen was heterozygous for X chromosome. The females in the pedigree have no phenotypic effect. They may be the carriers and their mating partners are normal and the male who are free of the disease do not pass it on. Male shows hemophilia. The homozygotic diseased female will not survive.

Three general types of hemophilia are known, each affecting the production of a prothrombin proteinase (factor X), an enzyme necessary for blood clotting.

(a) The ‘classic’ sex linked type, or hemophilia ‘A’ which can be detected by a reduction in the amount of a substance called antihemophilic factor (AHF or factor VIII) accounts for approximately 80 per cent of hemophilia.

(b) Hemophilia ‘B’ or Christmas disease is caused by reduction in the amount of plasma thromboplastin component (PTC or Factor IX) while accounts for 20 per cent of hemophiliacs. The gene for hemophilia ‘B’ is not allelic to that of hemophilia ‘A’.

(c) Hemophilia ‘C’ is due to the interaction of rare autosomal gene with the production of plasma thromboplastin antecedent (PTA or factor XI) is responsible for less than 1 per cent hemophiliacs.